This is really cool. tyvm. I'll give It a read through and try it out.
I have the "Hawaii" version. what did you happen to have? what kind of results?
The R9 290 and R9 390 are both Hawaii type of GPUs. The only difference is that the R9 390 has more memory and has higher factory OC but its mostly a rebrand.
The issue that I got with my R9 290 right before I sold it was that it mined at 30MH/s for about 3 years, from 2016 till 2018. Then for some reason it kept getting slower and slower, at first 29.5, then 29, then 28, then 27, etc, etc. Basically whenever the DAG got larger it slowed down, it looked like a DAG slowdown bug that affected the 280X and later the RX series before AMD offered a fix. However there was no fix for the R9 290 either.
So I just sold the GPU because it was a power hog, however that thread that is mentioned above has a user with the R9 290 and he gets 30MH/s by using a different miner or operating system. So read the thread and you will find out how to get 30MH/s with your R9 390. Just keep in mind its a power hog, mine used maybe 250 Watts or so.